The present invention concerns cooling fans, such as fans driven by and for use in cooling an industrial or automotive engine. More particularly, certain aspects of the invention relate to a ring fan, while other features concern fan blade design.
In most industrial and automotive engine applications, an engine-driven cooling fan is utilized to blow air across a coolant radiator. Usually, the fan is driven through a belt-drive mechanism connected to the engine crankshaft.
A typical cooling fan includes a plurality of blades mounted to a central hub plate. The hub plate can be configured to provide a rotary connection to the belt drive mechanism, for example. The size and number of fan blades is determined by the cooling requirements for the particular application. For instance, a small automotive fan may only require four blades having a diameter of only 9xe2x80x3. In larger applications, a greater number of blades are required. In one typical heavy-duty automotive application, nine blades are included in the fan design, the blades having an outer diameter of 704 mm.
In addition to the number and diameter of blades, the cooling capacity of a particular fan is also governed by the airflow volume that can be generated by the fan at its operating speed. This airflow volume is dependent upon the particular blade geometry, such as the blade area and curvature or profile, and the rotational speed of the fan.
As the cooling fan dimensions and airflow capacity increase, the loads experienced by the fan, and particularly the blades, also increase. In addition, higher rotational speeds and increased airflow through the fan can lead to de-pitching of the blades and significant noise problems. In order to address these problems to some degree, certain cooling fan designs incorporate a ring around the circumference of the fan. Specifically, the blade tips are attached to the ring, which provides stability to the blade tips. The ring also helps reduce vortex shedding at the blade tip, particularly when the ring is combined with a U-shaped shroud that follows the circumference of the ring.
The ring fan design, therefore, eliminates some of the structural difficulties encountered with prior unsupported cooling fan configurations. However, with the increased strength and improved vibration characteristics provided by the ring fan, the nominal operating conditions for these fans has been increased to again push the envelope of the ring fan""s capability. Moreover, the mass inertia of the circumferential ring increases the centripetal force exerted on the blade-ring interface.
Consequently, a need has again developed for ways to improve cooling airflow capacity of ring fans, while at the same time increasing their strength. This need becomes particularly acute as the operational rotational speeds of the fan increase to meet the increasing cooling demands for large industrial and automotive engines.
To address these needs, the present invention contemplates an engine driven cooling fan for use in an engine cooling system, in which the fan is a ring-type fan. The fan includes a central hub and a plurality of fan blades projecting radially outwardly from the hub, each of the blades having a blade root connected to the hub and a blade tip at an opposite end thereof. Each of the blades further defines a leading edge at an inlet side of the fan and a trailing edge at an outlet side of the fan. The cooling fan also includes a circumferential ring connected to the blade tip of each of the plurality of fan blades.
In one aspect of the invention, the circumferential ring includes a radially outwardly flared rim at the outlet side of the fan. The flared rim defines a flared surface adapted to nest over the circumferential rim of another cooling fan when the fans are stacked for storage or shipment. The flared rim decreases the height of a stack of a predetermined number of cooling fans, and increases the stability of the stack.
In another feature of certain embodiments of the present invention, each of the fan blades includes a support vane attached to the rear face of the blade. In the preferred embodiment, the support vane has a first end originating adjacent the root and the leading edge of the blade, and an opposite second end terminating at the trailing edge of the blade between the blade root and the blade tip. Preferably, the support vane is curved between the first end and the second end to follow the curvature of the airflow path along the rear face of the fan blade. With this feature, the support vane does not disrupt the airflow through the cooling fan.
The support vane originates at the blade root to provide additional support and stiffness to the fan blade at a critical region of the blade. More specifically, the location and configuration of the support vane increases the first vibration mode stiffness of the cooling fan so that the excitation frequency of the first mode exceeds the maximum rotational speed of the fan.
In a most preferred embodiment, each of the plurality of fan blades defines a blade length between the root and the tip and the support vane terminates at a position on the trailing edge in the first half of the blade length. This positioning again minimizes the effect of the support vane on the airflow through the cooling fan.
In another aspect of the cooling fan of the present invention, a circumferential support ring is provided at the central hub adjacent the blade root. With this feature, the support vane is attached to the support ring so that the ring adds support and stiffness to the support vane. Most preferably, the cooling fan further includes a vane support superstructure connected between the support ring and the support. This superstructure can include an arrangement of ribs connected between the ring and vane arranged to react the aerodynamic loads experienced by the support vane when the fan is operating at speed. This superstructure can include an angled rib projecting substantially perpendicularly from the support vane at a position substantially in the middle of the support vane. Since the vane is curved to follow the airflow path, the perpendicular rib will project at an angle relative to the blade root and support ring. Additional radial ribs can be provided closer to the leading edge of the blade.
In other embodiments, the cooling fan can also include a ring support superstructure connected between the support ring and the central hub. This ring superstructure provides support for the ring to assist it in reacting the loads applied to the support vane. Preferably, the ring superstructure includes an arrangement of ribs that correspond to the ribs of the vane support superstructure.
In another feature of the invention, the circumferential outer ring and the blade tip define a blend region therebetween. More specifically, this blend region is situated between the blade tip edge adjacent the trailing edge, and the flared rim of the circumferential ring. This blend region eliminates stress risers that ordinarily exist at the junction between the outer ring and the fan blades, which substantially reduces the risk of blade/ring separation. In addition, the inventive blend region can be accomplished in a typical molding process using a two-piece mold, without the need for inserts.
In yet another feature of the invention, each of the fan blades has a unique airfoil geometry that optimizes airflow characteristics while preserving blade strength and stiffness. Thus, one feature of the invention contemplates a blade geometry in which the blade camber varies along the radial length of the blade. More specifically, the camber has a minimum value at a position approximately one-sixth (⅙) of the radial length from the blade root. Thus, the camber decreases from the blade root to this position, and increases thereafter to the trailing edge of the blade. In alternative embodiments, the blade geometry also includes a chord angle that varies along the radial length of the blade, having a maximum value at the same position along the radial length. Similarly, the blade can define a variable chord-pitch-ratio (cpr) that has a maximum value at this same position. The resulting blade has improved airflow characteristics over prior known fan blades.
It is one object of the invention to provide a strength and performance optimized ring fan for an engine cooling system.
Another object resides in features that increase the stackability of the subject fan with other fans.
One benefit of the invention is that it provides a ring fan having increased first vibration mode stiffness. Another benefit is that this improved stiffness is accomplished without significant impact on the airflow characteristics of the fan.
Other objects and benefits of the present invention in its various embodiments will be appreciated upon consideration of the following written description and accompanying figures.